Static VAR (volt-ampere reactive) compensators (SVCs) are used to control voltage on AC transmission systems. Most SVCs include as one of the key controlling elements a thyristor-controlled reactor (TCR), as illustrated in FIG. 1A as a one-line diagram. The TCR consists of an anti-parallel pair of thyristor valves in series with an inductor. By delaying the firing of these thyristor valves beyond the natural conduction time of the inductive branch, a vernier control of the effective inductance applied to the AC power circuit is obtained. This is well established art.
Some applications of SVCs have shown a tendency towards interaction between the TCR currents and a second harmonic voltage distortion on the AC system. In many cases, the natural reaction of the TCR tends to reinforce the second harmonic voltage distortion. This effect has been traced to the natural tendency of the TCR to produce a DC component of current when a second harmonic voltage distortion is present. FIGS. 7A-7D illustrate this tendency. The DC component in turn affects the offset saturation experienced by the SVC transformer, as well as other transformers which may be in the vicinity. Since offset saturation in transformers is known to produce a second harmonic component of magnetizing current, a feedback loop is created, which involves both a DC and second harmonic component of current in the TCR.
This phenomenon was recognized by the power industry several years ago, and certain corrective measures have been installed. One such corrective measure is a large second harmonic power filter. Another is a special control function in the TCR which acts to null the measured DC component of current in the TCR. An example of this so-called "TCR current balancing control" is shown in FIG. 2. This control comprises an average detector 18 which measures the DC component of TCR current, an integrating regulator 19, which acts to null the measured DC component of TCR current, and a polarity switch 21 to properly account for operation in the two polarities of the AC voltage waveform. This signal is then added to the other regulator commands of the other phases "a, b and c" to obtain a total firing control signal for the TCR valves.
In prior art applications, this balancing control is applied to all three phases of the TCR. The three phases then operate on their own measured current and affect only their own firing control, with no significant information passed between phases Since nearly all TCRs are delta-connected, as shown in FIG. IB, there exist only two independent currents flowing into the AC transmission system. The third phase is simply the sum of the other two. Thus, the prior art creates a control system with three regulating state but only two independent paths of interacting with the AC transmission system.
An additional problem with the prior art is a tendency of the balancing control to oscillate as it converges. Yet another problem is poor transient response following a major disturbance on the AC transmission system.